Add subtitle toggle command on C key

[totimorpa]

Summary

• add a global C keyboard command to toggle subtitle visibility in the player
• reduce repeated "Subtitles loaded" notifications when subtitles are enabled via keyboard command
• preserve first-time keyboard-enable behavior

Implementation

• add application-level Action for C in main.qml
• on keyboard-enable, set a short-lived suppression flag for subtitle-related mpv prop-change events
• forward all other events unchanged

Why

• keyboard subtitle toggling works, but repeated subtitle toast popups are noisy during playback
• this keeps the command responsive while making repeated toggles less disruptive

Verification

• qmllint main.qml passes
• manual flow tested:
  1. play a video with subtitles available
  2. press C to toggle subtitles on/off
  3. subtitle toggle works
  4. repeated keyboard enables do not keep spamming subtitle toast

Scope

• one file only: main.qml
• no build-system or packaging changes

Tracking

• Related feature request: Desktop: Add keyboard subtitle toggle command (C) stremio-features#1766

[totimorpa]

Linked feature request: Stremio/stremio-features#1766

[totimorpa]

Hi @core1024 and @Ivshti, when you have a moment, could you please review this PR?

It adds subtitle toggle on the C key in desktop (main.qml) and reduces repeated keyboard-triggered subtitle toast spam.

Feature request tracked here: Stremio/stremio-features#1766

Thanks for your time.